A new study published in the PLoS One investigated the variability of predialysis concentrations of uremic toxins over time. This prospective study entailed the collection of predialysis serum samples from 18 stable chronic HD patients. Predialysis serum samples of the midweek session of week 0, 1, 2, 3, 4, 8, 12, and 16 were analyzed for a range of uremic toxins. It was observed that concentrations of the analyzed uremic toxins varied substantially between individuals, but also within stable HD patients (intra-patient variability). For urea, creatinine, beta-2-microglobulin, and some protein-bound uremic toxins, Intra-class Correlation Coefficient (ICC) was higher than 0.7. However, for phosphorus, uric acid, symmetric and asymmetric dimethylarginine, and the protein-bound toxins – hippuric acid and indoxyl sulfate, ICC values were below 0.7; indicating concentration variability within the individual patient even exceeding 65% of the observed inter-patient variability. From the findings, it was concluded that intra-patient variability may affect the interpretation of the association between a single concentration of certain uremic toxins and outcomes. It was suggested that during future outcome and interventional studies with uremic toxins, other than described here, intra-patient variability should be quantified and it should be taken into account that associations could be missed for solutes with a large intra-patient variability.